Usernames with capital letters force the Advanced filter mode

Problem

Editing a filter with assignee whose username is in capital letters disable the Basic search mode.

Steps to reproduce

1. Create a user with username in capital letters (for example, MEL), assign an issue to this user.
2. Create a filter in Basic search mode, select assignee with any user.
3. Save the filter.
4. Open the filter and edit it by changing the assignee to the user created in step 1.
5. Save the filter.

Expected result

When open the filter again, the filter will be available in Basic mode.

Actual result

The filter is only available in Advanced mode which the button for Basic mode is greyed out.